1. Field of the Invention:
The present invention relates to a bit holder block and cutter bit therefor of the type used particularly in mining operations. More particularly, the present invention relates to a bit holder block and cutter bit therefor of the aforesaid class which is structured to provide resistance to bit holder block cracking during mining operations.
2. Description of the Related Art:
Typical mining operations, such as the removal of coal along a seam, are performed by a mining apparatus which drives a rotary member provided with a plurality of cutting tips. The cutting tips engage a surface being mined to thereby cause removal of material from the surface. Conventional cutting tips are constructed of a hardened material, such as carbide. The cutting tip is connected to a conical head located at a forward end of an elongated rod, the combination collectively forming a cutter bit. Each cutter bit must be located in a predetermined orientation with respect to the rotary member of the mining apparatus. Because mining operations involve severe wear and tear on the cutter bit, it has become the practice to provide bit holder blocks for replaceably receiving, respectively, each of the cutter bits. It has further become the practice that because of the wear and tear experienced also by the bit holder blocks, that the bit holder blocks are structured for being replaceably connected with respect to the rotary member. Examples of replaceable bit holder blocks and cutter bits are described in U.S. Pat. No. 3,749,449 to Krekeler, U.S. Pat. No. 3,841,708 to Kniff et al., U.S. Pat. No. 3,992,061 to Rollins, U.S. Pat. No. 4,302,055 to Persson, U.S. Pat. No. 4,343,516 to Aden, and U.S. Pat. No. 4,415,208 to Goyarts.
In order for a bit holder block to provide a satisfactory service life, it must be structured so as to adequately handle the forces acting on it as a result of its cutter bit engaging a surface being mined. Ordinarily, the cutter bit is of an elongated cylindrical shape, and the bit holder block is provided with a cylindrical bore into which the cutter bit is received. The cutter bit is ordinarily retained at a predetermined abutting relationship with respect to the bit holder block via a first fastener. The bit holder block is ordinarily provided with surfaces which interface with cooperating surfaces on the rotary member for anchoring the bit holder block thereto in conjunction with one or more second fasteners or weldment.
As exemplified by the disclosure in Goyarts (cited hereinabove) and depicted in FIG. 7A, the conventional theory of force distribution on the cutter bit CB with respect to the bit holder block HB under mining loads was that the resultant cutter bit force F' was directed above the axis A of the cutter bit. This conventional theory presumed that the magnitude of the cutting force F.sub.c ' on the cutter bit was larger than the magnitude of the normal force F.sub.n ' on the cutter bit. Accordingly, prior bit holder block structures have been provided for accommodating this direction of the resultant cutting bit force F'.
However, applicants have observed the location and development of stress cracks in bit holder blocks after a period of usage. These field observations have led applicants to the realization that the conventional theory of the force distribution is in error because the magnitude of the normal force actually exceeds the magnitude of the cutting force. As indicated in FIG. 7B, it has been determined by applicants that in order to be correct, the theory of force distribution must properly take into account the correct relative magnitudes of the cutting and normal forces F.sub.n, F.sub.c on the cutter bit. An analysis of the direction and magnitude of these forces has led applicants to the realization that the true resultant cutter bit force F is directed below the axis A of the cutter bit.
Accordingly, the present invention is a bit holder block which provides a structure which takes into account the true resultant cutter bit force F.